（Ⅰ）若 AF=1，求证：CE∥平面 BDF；

（Ⅱ）若 AF=2，求平面 BDF 与平面 PCD所成的锐二面角的余弦值．

①除S外，上列元素的电负性从大到小依次为____．除K、S外第一电离能从大到小的顺序为____．

②在生成物中，A的晶体类型为____，含极性共价键的分子的中心原子轨道杂化类型为____．

③CN^﹣与N₂互为____，推算HCN分子中σ键与π键数目之比____．

阅读短文，回答问题。

Dear Amy,

   I want to 1 your friend. Let me introduce my family to you. My father is a doctor. He is tall 2 strong. My mother is a teacher. She is think and kind. My brother Sam is 6 years old. He's short and funny. We 3 in a big city. However, we usually go 4 on the weekends. I have a small and nice room. There is a bed and a desk in my room. I like reading books. I have many books 5 my desk. Welcome to my home.

                                                                                                                                                                        Yours,

                                                                                                                                                                          Lucy